1. Field of the Invention
The present invention relates to an offset rubber blanket for a grooveless or, ductless blanket cylinder for applying a printing image on sheet material or web material.
2. Description of the Related Art
Blankets which are used in offset printing as pressure blankets are usually composed of several different types of materials as well as various fabric components in order to increase the strength thereof. Natural rubber and synthetic rubber are used as blanket materials. A compressible rubber blanket additionally requires hard support surfaces which are not displaceable, or the blanket can be mounted on a blanket cylinder. Such a blanket cylinder has an axially extending groove or duct with clamping segments in which the opposite ends of the blanket are fastened. A blanket of excellent quality, correctly cut to size and well aligned, forms the basis of good offset printing. The printing image is applied on the blanket by means of a plate cylinder. However, it is not possible to produce endless images because of the blanket discontinuity resulting from the cylinder groove and the contact interruption between the cylinders which roll off on each other.
Moreover, vibrations result from the asymmetrical construction of the cylinders which roll off on each other and as a result of impacts at the groove. In addition, at high machine speeds, impact forces result from mass times acceleration which excite the natural or inherent bending forces of the cylinder, so that the printing quality is impaired. In addition, the loads due to vibration limit the printing widths if a certain weight of the revolving body is not to be exceeded.
For example, German Patent 27 00 118 C2 discloses a type of blanket in which an endless or continuous coating is applied on an exchangeable carrier sleeve of plastics material or a metal material. The coating is of an elastic material, preferably rubber. The rubber layer forms a tube which is completely free of gaps and seams and is tightly placed on the hollow body and its properties are identical to those of the above-described conventional blanket. The blanket sleeve manufactured in this manner is then mounted, for example, in accordance with the method described in German Patent 27 00 118 C2 or as known from flexo-printing, by sliding the blanket sleeve by means of compressed air over a printing cylinder core, i.e., the blanket cylinder, and subsequently fixing the blanket sleeve by switching off the supply of compressed air. However, the manufacture of such a blanket sleeve is very expensive. This is because the nickel and glass fiber-reinforced plastics material preferably used for the hollow body in order to achieve the necessary quality are comparatively expensive. In addition, compared to the manufacture of conventional blankets which can be continuously cut to size and processed in the plane state, the manufacture of the above-described blanket sleeve is very cumbersome because, the treating the circumference of a sleeve, each treatment step must be carried out individually, and in particular the rubber coating must be applied while the sleeve rotates.